Novel biopolymers and biopolymer blends, and method for producing same

ABSTRACT

Biopolymers and biopolymer blends, including block copolymers, prepared via enzyme-mediated catalysis preferably in a microorganism host in which the reaction conditions are selected to produce biopolymers and biopolymer blends having particular chemical compositions and microstructures.

STATEMENT AS TO FEDERALLY SPONSORED RESEARCH

[0001] This invention was funded pursuant to a grant received from theConsortium for Plant Biotechnology Research (Grant No. OR22072-77).Accordingly, the government may have rights in this invention.

BACKGROUND OF THE INVENTION

[0002] This invention relates to preparing biopolymers and biopolymerblends by enzyme-mediated catalysis e.g., in a microorganism host.

[0003] Biopolymers are polymers that can be prepared via enzyme-mediatedcatalysis either in vitro or in a microorganism host such as abacterium. These biopolymers are desirable because they arebiodegradable and biocompatible, making them suitable for a number ofapplications. In a typical procedure, one or more nutrients is fed tobacterial cells containing polymerase enzymes capable of processing thenutrients to form the desired biopolymer. The biopolymer is deposited inthe form of osmotically inert, intracellular granules that are thenextracted from the cells. This procedure has been used, for example, toprepare homopolymers and random copolymers containing 3-hydroxyorganoate units.

SUMMARY OF THE INVENTION

[0004] The inventors have discovered how to control the microstructureand chemical composition of biopolymers and biopolymer blends producedvia enzyme-mediated catalysis. Accordingly, it is now possible toprepare biopolymers and biopolymer blends that are tailored to meet theneeds of a specific application. In particular, block copolymers andphase-separated blends can be prepared.

[0005] The phase-separated blends feature particles having abiopolymer-containing core substantially surrounded by abiopolymer-containing shell in which the core and shell have differentchemical compositions. The biopolymers may be in the form ofhomopolymers, copolymers, or combinations thereof, where “homopolymer”refers to a polymer having a single type of monomer unit and “copolymer”refers to a polymer having two or more different monomer units. Thethickness of the shell is less than about 1 micrometer, preferably lessthan about 0.1 micrometer, and more preferably less than about 0.05micrometer.

[0006] A number of different core/shell combinations can be prepared.According to one embodiment, the core includes a homopolymer and theshell includes a copolymer, preferably in which the copolymer andhomopolymer have common monomer units. Alternatively, the core caninclude the copolymer and the shell can include the homopolymer. Theparticle can further include one or more additional shell layers.

[0007] Particularly useful biopolymers are homopolymers and copolymersthat include hydroxy organoate units. Such units have the generalformula:

[0008] where R is a group containing between 1 and 30 carbon atoms,inclusive, and n is at least 1. Examples include 3-hydroxy organoateunits (n=1) and 4-hydroxy organoate units (n=2). Specific examplesinclude 3-hydroxy butyrate (R=methyl; n=1), 3-hydroxy valerate (R=ethyl;n=1), and combinations thereof. In one embodiment, the core includes ahomopolymer containing 3-hydroxy butyrate units and the shell includes acopolymer containing 3-hydroxy butyrate and 3-hydroxy valerate units. Inanother embodiment, the core includes a copolymer containing 3-hydroxybutyrate and 3-hydroxy valerate units, and the shell includes ahomopolymer containing 3-hydroxy butyrate units.

[0009] Block copolymers having at least two blocks can also be prepared.Preferably, the blocks contain 3-hydroxy organoate units. For example,the first block may contain 3-hydroxy butyrate units and the secondblock may contain 3-hydroxy valerate units.

[0010] The biopolymers and biopolymer blends may be prepared usingmicroorganism hosts by controlling the nutrients available to themicroorganism, resulting in the sequential formation of differentpolymers, or polymer blocks, within the same granule. In the case ofblock copolymers, for example, the relative amounts of the nutrients andthe timing of introduction into the microorganism may be selected suchthat the first nutrient is available for reaction substantiallythroughout the process and the second nutrient is available for reactionthroughout selected portions of the process.

[0011] The biopolymers and biopolymer blends are useful in a number ofapplications. For example, they may be compounded with a tackifier and,optionally, a crosslinking agent to form an adhesive composition.Examples of suitable tackifiers and crosslinking agents are described,e.g., in Rutherford et al., U.S. Pat. No. 5,614,576 and Rutherford etal., U.S. Pat. No. 5,753,364, both of which are hereby incorporated byreference. It is also possible to combine the biopolymers and biopolymerblends with another polymer matrix to modify the properties of thepolymer matrix. For example, the two phase biopolymer particles can beincorporated into a brittle polymer such as polystyrene to improve theimpact resistance of the brittle polymer.

[0012] Other features and advantages will be apparent from the followingdescription of preferred embodiments thereof, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a graph of carbon source concentration vs. time for theproduction of block copolymers from fructose and 3-hydroxy valeric acidstarting materials.

[0014]FIGS. 2A-2D are a series of microphotographs obtained fromtransmission electron microscopy throughout the course of the reactioninvolving fructose and 3-hydroxy valeric acid starting materials thatultimately yielded polymer particles having copolymer core surrounded bya homopolymer shell.

DETAILED DESCRIPTION

[0015] Biopolymers and biopolymer blends according to the invention arepreferably produced from microorganisms via enzyme-mediated catalysisunder nutrient-limited conditions. The microorganism serves as abioreactor. The microorganism is selected based upon the desiredbiopolymer. Specifically, microorganisms having polymerases suitable forcatalyzing the reaction of monomers to produce the desired biopolymerare selected. The polymerases may be native to the particularmicroorganism. Alternatively, the microorganism can be geneticallyengineered using known techniques to contain the desired polymerases.

[0016] One class of useful microorganisms includes bacteria. Specificexamples include Pseudomonas aeruginosa, Ralstonia eutropha (formerlyAlcaligenes eutrophus), Rhodospirillum rubrum, and Bacillus megaterium.Each of these bacteria is capable of metabolizing nutrient sourcesincluding alkanes, alkanols, organoic acids, alkenes, alkenols, alkenoicacids, and esters, for example, to produce biopolymers such aspoly(hydroxy organoates). R. eutropha is particularly useful forreacting 3-hydroxy butyric acid and 3-hydroxy organoic acid monomershaving an odd number of carbon atoms such as 3-hydroxy valeric acid, aswell as precursors thereof, to form poly(3-hydroxy organoate) polymersand copolymers.

[0017] To prepare the biopolymer or biopolymer blend, appropriatestarting materials are introduced into the microorganism. In the case ofpoly(hydroxy organoate) biopolymers, the starting materials may includehydroxy organoic acids or precursors thereof. An example of a suitableprecursor is fructose which the microorganism metabolizes to form3-hydroxy butyric acid monomer.

[0018] The chemical composition and microstructure of the biopolymerproduct are influenced by the relative amounts of starting materials,the concentration of microorganisms, and the timing of nutrientintroduction into the bioreactor (i.e., the point at which the startingmaterial is added to the microorganism). For example, in the case of areaction involving fructose (as a source of 3-hydroxy butyric acid) andan odd chain fatty acid such as 3-hydroxy valeric acid, if fructose isfed with a limited amount of the valeric acid monomer, poly(3-hydroxyvalerate) will no longer be produced after exhaustion of the valerate,resulting in a polymer particle featuring a poly(3-hydroxy(butyrate-co-valerate)) copolymer core substantially surrounded by apoly(3-hydroxy butyrate) homopolymer shell. On the other hand, supplyingthe valeric acid monomer halfway through particle synthesis on fructoseresults in a polymer particle featuring a poly(3-hydroxy butyrate)homopolymer core substantially surrounded by a poly(3-hydroxy(butyrate-co-valerate)) copolymer shell. By increasing cell densityand/or decreasing the amount of valeric acid fed to the bacteria, it ispossible to stop copolymer synthesis within a polymer chain, resultingin the formation of block copolymers having a poly(3-hydroxy butyrate)block and a poly(3-hydroxy (butyrate-co-valerate)) copolymer block.

[0019] A theoretical model has been developed to enable the experimentalconditions to be designed for production of a given biopolymer. Themodel (1) describes the experimentally observed molecular weightdistribution in a culture of biopolymer-producing cells, and (2)predicts transient synthesis situations encountered during the proposedpolymer synthesis process. The model can be used, for example, topredict and delineate the conditions that lead to the formation of blockcopolymers.

[0020] The model assumes that polymerase molecules sit on the surface ofpolymer granules and catalyze the addition of monomer units to thegrowing (“elongating”) polymer chain that is aggregating into or ontothe granule. After initiation, chains are elongating until anirreversible termination step detaches the chain from the polymeraseenzyme. Given a constant supply of monomer units, the polymer synthesisis governed by three rates describing initiation, elongation, andtermination of polymer chains. The model results in the followingpopulation balance that describes generally the dynamics of themolecular weight distribution of polymer chains that changes in time t:$\begin{matrix}{{{\frac{\partial{A\left( {x,t} \right)}}{\partial t} + {\frac{\partial}{\partial x}\left\lbrack {{R_{el}\left( {x,S} \right)}{A\left( {x,t} \right)}} \right\rbrack} + {{R_{t}\left( {x,S} \right)}{A\left( {x,t} \right)}}} = 0}{\frac{\partial\left( {x,t} \right)}{\partial t} = {{R_{t}\left( {x,S} \right)}{I\left( {x,t} \right)}}}} & (1)\end{matrix}$

[0021] where A(x,t) and I(x,t) are the molecular weight distribution ofactive and inactive chains, respectively; R_(el) is the elongation rate;R_(i) is the initiation rate; and R_(t) is the termination rate. In thegeneral case, the initiation and elongation rates depend on themolecular weight “x” of a chain and on the monomer concentration “S”.The initiation rate contributes to the physical picture through theboundary condition.

[0022] Assuming a steady state synthesis, equation (1) can be solved forthe active chain distribution on the basis of the measurable and timeinvariant overall distribution w(x), representing the sum of A(x) andI(x), using the relationship: $\begin{matrix}{{A(x)} = {\frac{1}{R_{el}} \cdot \frac{N_{A}R_{p}}{\int_{x\quad \min}^{x\quad \max}{{{xw}(x)}{x}}} \cdot {\int_{x}^{x\quad \max}{{w(y)}{y}}}}} & (2)\end{matrix}$

[0023] where N_(A) is Avogadro's number. After determining the activechain distribution A(x), the termination rate R_(t) is determined usingthe following equation: $\begin{matrix}{{R_{t}(x)} = {\frac{1}{A(x)} \cdot \frac{N_{A}R_{P}{w(x)}}{\int_{x\quad \min}^{x\quad \max}{{{xw}(x)}{x}}}}} & (3)\end{matrix}$

[0024] Gel permeation chromatography can be used to measure the timeinvariant molecular weight distribution for different synthesisconditions. This value can then be used to estimate the unknown activechain distribution and termination rate. Specifically, the former can bedirectly computed from the measured molecular weight distribution usingEquation (2), while the latter as a function of molecular weight ofchains, can be obtained using Equation (3). Once the model parametershave been identified, the steady state molecular weight distribution canbe computed by solving Equation (1).

[0025] This model has been used to design experiments that would produceblock copolymers in R. eutropha using, as starting materials, fructoseand 3-hydroxy valeric acid. The experimental conditions predicted toyield block copolymers in this reaction are shown in FIG. 1. The graphin FIG. 1 plots carbon source concentration vs. time. According to thegraph, if valeric acid consumption is faster than the time required tosynthesize a polymer chain, block copolymers result. The residence timeof valeric acid in the media can be controlled by the cell concentrationand the initial valeric acid concentration.

[0026] The invention will now be described further by way of thefollowing example.

EXAMPLE

[0027] This example describes the preparation and characterization ofpolymer granules featuring a core containing the copolymerpoly(3-hydroxy (butyrate-co-valerate)) and a shell containingpoly(3-hydroxy butyrate) homopolymer. Throughout the example, “PHA”refers to poly(3-hydroxyorganoate), “PHB” refers to poly(3-hydroxybutyrate), “PHV”refers to poly(3-hydroxy valerate), and “PH(B-co-V)”refers to poly(3-hydroxy (butyrate-co-valerate)).

[0028] Materials

[0029] Epon-Araldite epoxy resin and 200-mesh copper grids fortransmission electron microscopy (TEM) were purchased from Ted-Pella(Reading, Calif.). Homo-PHB samples were received from Monsanto.PH(B-co-V) (49% HV) was prepared in a bioreactor (conditions presentedbelow) with R. eutropha grown on 10 g/l fructose and 1 g/l valeric acidfor 24 h. All other media components and chemicals were purchased fromSigma (St. Louis, Mo.).

[0030] Bacterial Growth

[0031]R. eutropha H16 (ATCC 17699) was used in all polymer productionexperiments. Inocula were grown in 4-1 shake flasks with 1.51 of mineralsalts medium with 30 g/l fructose and 4 g/l (NH₄)₂SO₄ to an OD₄₃₆ of 4.0measured on a spectrophotometer (Model 8452A, Hewlett Packard, Avondale,Pa.). Cells were harvested in exponential growth (μ=0.33/h) and washedtwice with sterile phosphate buffer (0.036 M, pH 7.0). Cells were theninoculated into 1.51 of minimal media (˜0.4 g/l) containing 0.02 g/l(NH₄)₂SO₄ and the indicated carbon source/s (10 g/l fructose, 1 g/lvalerate). Nitrogen was included to facilitate adaptation of the cultureto new media conditions containing fatty acids. The low nitrogen levelscould only be used to produce approximately 6 mg/l of biomass and thuscontributed insignificantly to growth at the cell density used. Reactorswere operated at 700 rpm, 30° C., pH 7 (controlled with 1M NaOH and 4%H₃PO₄), and 1 vvm air flow; dissolved oxygen remained above 90%.

[0032] Gas Chromatography (GC) Analysis of PHAs

[0033] All polymer samples were analyzed by propanolysis and subsequentGC analysis according to standard protocols described in Riis et al., J.Chromatogr. (1988) 445:285.

[0034] Substrate Analysis

[0035] Fructose levels in the media were measured using aD-Glucose/D-Fructose Enzymatic BioAnalysis kit (Boehringer Mannheim,Mannheim, Germany). Valeric acid concentrations were measured using gaschromatography (GC). GC vials were filled with 0.85 ml of samplesupernatant. A total of 75 μl of a 1-N HCl solution with 0.2 g/lisovaleric acid was added to acidify the samples. The isovaleric acidserved as an internal standard. Samples were run on a DB-FFAP columnwith the following temperature profile: 100° C. for 5 min, ramp at 10°C./min, final temperature of 200° C. for 5 min (J&W Scientific, Folsom,Calif.). Retention times were approximately 10.5 min for isovalerate and11.5 min for valeric acid. A calibration curve was established for eachset of samples.

[0036] Granule Isolation

[0037] Harvested cells were concentrated, by centrifugation,approximately 100-fold. The cell slurry was passed through a FrenchPress twice at a cell pressure of 10,000 psi. After settling overnight,150 μl of settled cell slurry containing the granules was added to 15 mlof a 1% Triton surfactant solution at pH 13 and agitated at roomtemperature for 15 min. Granules were centrifuged at 4000×g for 15 minand washed twice with distilled water. Granules were dried overnight atroom temperature under vacuum to remove all of the water.

[0038] Differential Scanning Calorimetry (DSC)

[0039] DSC was used to determine if the granules were layered structureshaving two distinct phases. DSC analysis was carried out on a liquidnitrogen cooled DSC (Pyris model, Perkin-Elmer, Norwalk, Conn.). Sampleswere taken from the bioreactor at 6, 12, 18, and 24 h. Granules wereisolated from the bacteria as described above. For glass transitionmeasurements, 9-10 mg samples were used. The temperature was ramped to200° C., held for 1 min, followed by slow quenching at a rate of −20°C./min to a final temperature of −60° C. A rate of 20° C./min from −60°C. to 30° C. was used to analyze glass transition temperatures.

[0040] TEM Preparation

[0041] TEM measurements were also used to determine if the granules werelayered structures having two distinct phases. As in the case of the DSCmeasurements, samples were taken from the bioreactor at 6, 12, 18, and24 h. Dried granules were fixed in Epon-Araldite epoxy for 2 days at 60°C. The samples were cryo-microtomed at −20° C. to form a smooth face ofexposed polymer. Each epoxy block was stained with RuO₄ vapor. Specimenswere taped to the lid of a 5-ml glass vial containing 0.02 g RuCl₃-xH₂Oand 1 ml NaOCl (10-13%), sealed and stained for 7.5 h. After staining,the samples were removed and stored in a hood to dry.

[0042] Staining sufficiently hardened the samples so that cryo-microtomywas no longer necessary. Samples were cut 70 nm thick as close to thestained surface as possible. Increased depth of cutting gave slices withdecreased staining, allowing a desired contrast to be achieved bychanging the cutting depth. Sections were placed on 200-mesh coppergrids and viewed in an electron microscope at 60 kV (JEM1200ex2, Joel,Boston, Mass.).

[0043] Results

[0044] PH(B-co-V) was produced during the first 12 h. of the reaction.The PHV and PHB synthesis rates were 0.030 g PHV/l per h and 0.033 gPHB/l per h, giving a copolymer composition of 48% HV. The valerate wasexhausted at approximately 12 h at a rate of 0.0894 g/l per h afterwhich only PHB was produced at 0.033 g PHB/l per h from the remainingfructose, which was utilized at a constant rate of 0.0861 g/l per hthroughout all 24 h of the experiment. The concentration of PHV in thereactor remained constant during the last 12 h of the experiment,suggesting that no degradation of polymer occurs concomitantly with newpolymer synthesis.

[0045] DSC measurements of the isolated granules confirmed the presenceof two ploymer phases in the granules. Glass transition temperatures ofthe individual components PH(B-co-V) (49% HV) and PHB were determined tobe −5° C. and 5° C., respectively. Each sample throughout the course ofthe reaction showed two distinct glass transitions, indicating thepresence of two phases. Table I shows the weight percent of each type ofpolymer in the granule at various stages during the course of thereaction. All amounts are based upon percentage of total polymer weight.“% CDW” refers to the polymer percentage of cell dry weight. TABLE ITime (h) % CDW % PHB % PH(B-co-V) 0 21 100 0 6 53 23 77 12 59 13 87 1866 31 69 24 66 41 59

[0046] The results shown in Table I show that during the first 12 h,only PH(B-co-V) was being produced. Nevertheless, two glass transitionswere observed because of the PHB stored during inoculum growth, prior tointroduction into the reactor. After 12 h, only PHB is formed. The 5° C.transition was observed to increase with the increasing PHB weightfraction of the sample because glass transition heatflow changes aredirectly proportional to the mass of that component in the sample.

[0047] TEM imaging confirmed the presence of layered granules consistingof a copolymer core and a homopolymer shell. The results are shown inFIGS. 2A-2D. PHB regions are shown as light gray regions, whilePH(B-co-V) regions are dark gray regions. Referring to FIGS. 2A-2D, someregions of PHB were seen in the 6 and 12 h samples because of the PHBstored during inoculum growth. The granules of PH(B-co-V) seemed tocoalesce, making it difficult to discern individual granule boundaries.The PH(B-co-V) granules likely coalesce much easier than the PHBgranules because of their lower crystallinity. At 18 and 24 h, layeredgranule structures became clearly visible (FIGS. 2C and 2D).

[0048] Other embodiments are within the following claims.

What is claimed is:
 1. A polymer particle comprising: (a) a corecomprising a first biopolymer; and (b) a shell substantially surroundingsaid core comprising a second biopolymer different from said firstbiopolymer, said shell having a thickness that is less than about 1micrometer.
 2. A polymer particle according to claim 1 wherein saidshell has a thickness that is less than about 0.1 micrometer.
 3. Apolymer particle according to claim 1 wherein said shell has a thicknessthat is less than about 0.05 micrometer.
 4. A polymer particle accordingto claim 1 wherein said first biopolymer and said second biopolymercomprise hydroxy organoate units.
 5. A polymer particle according toclaim 4 wherein said hydroxy organoate units are selected from the groupconsisting of 3-hydroxy organoates, 4-hydroxy organoates, andcombinations thereof.
 6. A polymer particle according to claim 4 whereinsaid hydroxy organoate units are selected from the group consisting of3-hydroxy butyrate, 3-hydroxy valerate, and combinations thereof.
 7. Apolymer particle according to claim 4 wherein said first biopolymercomprises 3-hydroxy butyrate units.
 8. A polymer particle according toclaim 4 wherein said second biopolymer comprises 3-hydroxy butyrateunits.
 9. A polymer particle according to claim 4 wherein said firstbiopolymer comprises 3-hydroxy valerate units.
 10. A polymer particleaccording to claim 4 wherein said second biopolymer comprises 3-hydroxyvalerate units.
 11. A polymer particle according to claim 1 wherein saidfirst biopolymer is a homopolymer and said second biopolymer is acopolymer.
 12. A polymer according to claim 11 wherein said secondbiopolymer is a block copolymer.
 13. A polymer particle according toclaim 11 wherein said first biopolymer is a homopolymer comprising3-hydroxy butyrate units and said second biopolymer is a copolymercomprising 3-hydroxy butyrate and 3-hydroxy valerate units.
 14. Apolymer particle according to claim 1 wherein said first biopolymer is acopolymer and said second biopolymer is a homopolymer.
 15. A polymerparticle according to claim 14 wherein said first biopolymer is a blockcopolymer.
 16. A polymer particle according to claim 14 wherein saidfirst biopolymer is a copolymer comprising 3-hydroxy butyrate and3-hydroxy valerate units, and said second biopolymer is a homopolymercomprising 3-hydroxy butyrate units.
 17. A polymer particle comprising:(a) a core comprising a first biopolymer comprising units selected fromthe group consisting of 3-hydroxy butyrate, 3-hydroxy valerate, andcombinations thereof; and (b) a shell substantially surrounding saidcore comprising a second biopolymer comprising units selected from thegroup consisting of 3-hydroxy butyrate, 3-hydroxy valerate, andcombinations thereof, wherein said second biopolymer is different fromsaid first biopolymer.
 18. A polymer particle according to claim 17wherein said first biopolymer is a homopolymer and said secondbiopolymer is a copolymer.
 19. A polymer particle according to claim 18wherein said first biopolymer is a homopolymer comprising 3-hydroxybutyrate units and said second biopolymer is a copolymer comprising3-hydroxy butyrate and 3-hydroxy valerate units.
 20. A polymer particleaccording to claim 17 wherein said first biopolymer is a copolymer andsaid second biopolymer is a homopolymer.
 21. A polymer particleaccording to claim 20 wherein said first biopolymer is a copolymercomprising 3-hydroxy butyrate and 3-hydroxy valerate units, and saidsecond biopolymer is a homopolymer comprising 3-hydroxy butyrate units.22. A block copolymer comprising: (a) a first block comprising hydroxyorganoate units; and (b) a second block comprising hydroxy organoateunits, wherein said first block is different from said second block. 23.A block copolymer according to claim 22 wherein said hydroxy organoateunits are selected from the group consisting of 3-hydroxy organoates,4-hydroxy organoates, and combinations thereof.
 24. A block copolymeraccording to claim 22 wherein said first block comprises 3-hydroxybutyrate units and said second block comprises 3-hydroxy valerate units.25. A block copolymer according to claim 22 further comprising a thirdblock comprising hydroxy organoate units.
 26. A process for preparing apolymer particle comprising introducing first and second nutrients intoa microorganism comprising a polymerase capable of processing saidnutrients to form biopolymers, wherein the relative amounts of saidnutrients and the timing of introduction into said microorganism areselected to produce a polymer particle comprising: (a) a core comprisinga first biopolymer; and (b) a shell substantially surrounding said corecomprising a second biopolymer different from said first biopolymer,said shell having a thickness that is less than about 1 micrometer. 27.A process according to claim 26 wherein said microorganism comprises abacterium.
 28. A process according to claim 26 wherein said bacteriumcomprises R. eutropha.
 29. A process according to claim 26 wherein saidfirst nutrient comprises a hydroxy organoic acid, or a precursorthereof, and said second nutrient comprises a hydroxy organoic acid, ora precursor thereof, that is different from said first nutrient.
 30. Aprocess according to claim 29 wherein said first nutrient comprises3-hydroxy butyric acid, or a precursor thereof, and said second nutrientcomprises 3-hydroxy valeric acid, or a precursor thereof.
 31. A processaccording to claim 30 wherein said first nutrient comprises fructose.32. A process according to claim 26 wherein the relative amounts of saidnutrients and the timing of introduction into said microorganism areselected to produce a polymer particle comprising: (a) a core comprisinga first biopolymer in the form of a homopolymer; and (b) a shellsubstantially surrounding said core comprising a second biopolymer inthe form of a copolymer.
 33. A process according to claim 32 whereinsaid first nutrient comprises a hydroxy organoic acid, or a precursorthereof, and said second nutrient comprises a hydroxy organoic acid, ora precursor thereof, that is different from said first nutrient.
 34. Aprocess according to claim 33 wherein said first nutrient comprises.3-hydroxy butyric acid, or a precursor thereof, and said second nutrientcomprises 3-hydroxy valeric acid, or a precursor thereof, said polymerparticle comprising: (a) a core comprising a homopolymer comprising3-hydroxy butyrate units; and (b) a shell substantially surrounding saidcore comprising a copolymer comprising 3-hydroxy butyrate and 3-hydroxyvalerate units.
 35. A process according to claim 26 wherein the relativeamounts of said nutrients and the timing of introduction into saidmicroorganism are selected to produce a polymer particle comprising: (a)a core comprising a first biopolymer in the form of a copolymer; and (b)a shell substantially surrounding said core comprising a secondbiopolymer in the form of a homopolymer.
 36. A process according toclaim 35 wherein said first nutrient comprises a hydroxy organoic acid,or a precursor thereof, and said second nutrient comprises a hydroxyorganoic acid, or a precursor thereof, that is different from said firstnutrient.
 37. A process according to claim 36 wherein said firstnutrient comprises 3-hydroxy butyric acid, or a precursor thereof, andsaid second nutrient comprises 3-hydroxy valeric acid, or a precursorthereof, said polymer particle comprising: (a) a core comprising acopolymer comprising 3-hydroxy butyrate and 3-hydroxy valerate units;and (b) a shell substantially surrounding said core comprising ahomopolymer comprising 3-hydroxy butyrate units.
 38. A process forpreparing a block copolymer comprising introducing first and secondnutrients into a microorganism comprising a polymerase capable ofprocessing said nutrients to form biopolymers, wherein the relativeamounts of said nutrients and the timing of introduction into saidmicroorganism are selected to produce a block polymer.
 39. A processaccording to claim 38 wherein the relative amounts of said nutrients andthe timing of introduction into said microorganism are selected suchthat said first nutrient is available for reaction substantiallythroughout the process and said second nutrient is available throughoutselected portions of the process to produce said block copolymer.
 40. Aprocess according to claim 38 wherein said microorganism comprises abacterium.
 41. A process according to claim 40 wherein said bacteriumcomprises R. eutropha.
 42. A process according to claim 38 wherein saidfirst nutrient comprises a hydroxy organoic acid, or a precursorthereof, and said second nutrient comprises a hydroxy organoic acid, ora precursor thereof, that is different from said first nutrient.
 43. Aprocess according to claim 42 wherein said first nutrient comprises3-hydroxy butyric acid, or a precursor thereof, and said second nutrientcomprises 3-hydroxy valeric acid, or a precursor thereof.
 44. A processaccording to claim 43 wherein said first nutrient comprises fructose.45. An adhesive composition comprising: (A) a plurality of polymerparticles, each of which comprises: (a) a core comprising a firstbiopolymer; and (b) a shell substantially surrounding said corecomprising a second biopolymer different from said first biopolymer,said shell having a thickness that is less than about 1 micrometer; and(B) a tackifier.
 46. An adhesive composition according to claim 45wherein said composition is crosslinked.
 47. An adhesive compositioncomprising: (A) a block copolymer comprising: (a) a first blockcomprising 3-hydroxy organoate units; and (b) a second block comprising3-hydroxy organoate units, wherein said first block is different fromsaid second block; and (B) a tackifier.
 48. An adhesive compositionaccording to claim 47 wherein said composition is crosslinked.